The present invention relates to devices for measuring and/or monitoring body fluid compartmentalization and in particular to devices that utilize multiple frequencies and body impedance and/or resistance and/or capacitive reactance measurements to determine body water volume and the intracellular versus extracellular water distribution.
The body's water content and its distribution between intracellular and extracellular compartments are biologically and pathologically significant and an accurate determination thereof can be used to improve medical care. The present derivation of these parameters has been made using laboratory measurement techniques commonly referred to as the "gold standard". Due to the time and expense involved in effecting such measurements, the measurements are not routinely made, and valuable diagnostic information is frequently not available to the physician.
For example, while heart failure is diagnosed and treated properly in the majority of patients, there are patients who are not properly diagnosed. These patients include those who show early signs of myocardial dysfunction, but do not exhibit tell-tale clues, and consequently, treatment is withheld. Others include symptomatic patients whose condition is incorrectly diagnosed as heart failure. Similar errors in diagnosis apply to disorders usually characterized by hypovolemia or total body water loss. Again, because of the known difficulty in accurately assessing a patient's hemodynamic and fluid status by physical examination alone, physicians rely on a number of calculations and special studies to assist in their diagnosis. These aids include frequent sampling of serum and urine for determination of osmolality, electrolyte concentration, and renal function studies, and placement of indwelling balloon-tipped catheters in a patient's pulmonary artery. Measurements obtained with such catheters may influence therapy, but in some cases the catheters produce serious complications in the patient. Such procedures are still of unproven utility in improving overall patient morbidity and mortality and are fraught with difficulties in interpretation.
In the medical literature, there is sufficient information to show that bioelectrical impedance determination can provide useful information in patient diagnoses. There is little doubt that in healthy individuals, single-frequency bioelectric impedance determinations correlate closely with total body water stores. However, there is a problem with reliability and accuracy when this method is used to evaluate body water stores in individuals who are not healthy. These problems are caused by applying data from healthy individuals to the evaluation of individuals having disorders involving water homeostasis, when one of the underlying assumptions made in the study of healthy individuals no longer holds true. Specifically, healthy individuals have a constant relationship between total body water and extracellular water. Individuals with diseases such as heart disease, renal disease, liver disease, malnutrition, and severe dehydration have an unpredictable ratio of total body water to extracellular water.
In prior attempts to measure water compartmentalization, researchers have noted a frequency dependent variation in tissue impedance. Reference is made to U.S. Pat. No. 3,316,896, entitled "Apparatus and Methods for the Measure of the Electrical Impedance of Living Organisms," issued to Thomasset, U.S. Pat. No. 3,949,736 entitled "Circuit for Automatically Deriving and Measuring Relative Voltages Associated with Impedance Components of a Biological Object," issued to Vrana et al., U.S. Pat. No. 3,971,365 entitled "Bioelectrical Impedance Measuring System," issued to Smith, and U.S. Pat. No. 4,155,351 entitled "Medical Instrument for Detecting Body Impedance," issued to Teshima et al., which disclose information regarding the frequency dependent variation in tissue impedance.
In a given tissue, the magnitude of the frequency-dependent impedance change correlates with the ratio of extracellular water to total water. Prior investigators suggest that the total water measurement is best predicted at low frequencies, but our studies show that low frequency measurements largely ignore the intracellular water and, thus, are accurate only when the intracellular to extracellular water ratio is constant. Previous efforts have been made to establish a relationship between a subject's frequency-related bioelectric impedance, measured total body water and intracellular or extracellular water. To date, these efforts have failed to obtain mathematical correlations sufficiently good for use in clinical medicine. However, these efforts have produced results sufficient to establish rough guidelines and to indicate predictive differences in health and disease. At the present time, published equations for determining the compartmentalization of body water have not been fully developed.
Conventional approaches employed in determining the water content of the body as well as its intracellular and extracellular water distribution are described in U.S. Pat. No. 4,008,712, entitled "Method of Monitoring Body Characteristics," issued to Nyboer, an article entitled "Measurement of Transcellular Fluid Shift During Haemodialysis," by Meijer et al., in Medical & Biological Engineering & Computing, page 147, March 1989, and in "Electrical Measurement of Fluid Distribution in Human Legs: Estimation of Extra- and Intra-Cellular Fluid Volume," by Kanai et al., in Journal of Microwave Power, Vol. 18 , No. 3, page 233 (1983).
The Kanai et al. article illustrates a system that is adapted to monitor relative shifts in body fluids, which system is shown in FIG. 6 thereof. Kanai et al. describe measurements of water compartment shifts, in relative terms, monitored in the frequency range of 1-100 KHz. Very little is described regarding the specifics of the circuitry employed to monitor body impedance.
Accordingly, there exists a need for a system that accurately and repeatably measures body impedance over a range of frequencies and combines this information with anthropometric data and empirically derived relationships to provide clinically usable data for monitoring body water volume and compartmentalization in both absolute and relative terms.